As materials for next-generation power semiconductor devices, a GaN (gallium nitride) based semiconductor has been expected. A GaN based semiconductor device has a wide band gap and can realize a high breakdown voltage and low loss, as compared with a device of Si.
Generally, in a transistor of the GaN based semiconductor, a high electron mobility transistor (HEMT) using two-dimensional electron gas (2DEG) as carriers is adopted. However, the normal HEMT becomes a normally-on transistor in which current flows when a voltage is not applied to a gate. It is difficult to realize a normally-off transistor for HEMT. In particular, it is difficult to realize the normally-off transistor having a high threshold voltage.